1. Technical Field
The present invention relates to an ink composition, comprising a prescribed concentration or more of lithium ions, capable of ensuring superior printing quality by preventing clogging and the like as a result of preventing or inhibiting the precipitation of insoluble or poorly soluble salts in water/organic solvent occurring due to highly fatty acids and salts thereof contained in a plastic container or member such as an ink cartridge when the ink composition has contacted the plastic container or member.
2. Related Art
Inkjet recording is a method for recording letters or images (to also be simply referred to as “images”) on the surface of a recording medium by discharging an ink composition from a micronozzle in the form of small droplets. Examples of practical inkjet recording methods include a method in which letters and images are recorded onto the surface of a recording medium by converting an electrical signal to a mechanical signal using a piezoelectric device and intermittently discharging an ink composition stored in a nozzle head portion, and a method in which letters and images are recording onto the surface of a recording medium by generating bubbles by rapidly heating a portion closest to a discharge portion and intermittently discharging an ink composition stored in a nozzle head portion utilizing the volumetric expansion generated by the bubbles.
In addition, although ink compositions for inkjet recording typically have various types of dyes dissolved in water, organic solvent or mixture thereof in consideration of safety and printing characteristics, stricter conditions are required with respect to various characteristics as compared with ink compositions for writing instruments in the manner of fountain pens and ballpoint pens.
In recent years in particular, inkjet printers have come to be employed to produce printed articles for advertising, and even stricter requirements are applicable when forming color images using a plurality of ink compositions. In addition to ink compositions used to form color images being required to have satisfactory color development, they are also required to have basic characteristics such as the absence of bronzing when printing onto glossy paper and the absence of discoloration and fading during subsequent storage of printed articles. In addition, since printed articles may be placed both indoors and outdoors, they are exposed not only to sunlight but also various light and outside air (such as ozone, nitrogen oxides and sulfur oxides), and ink compositions have been developed that have superior light resistance and gas resistance. These characteristics are heavily dependent on the compound used for the colorant.
However, since the water solubility of the colorant itself tends to be low in ink compositions having superior light resistance and gas resistance, not only is it difficult to obtain stable discharge properties, but particularly in the case cyan-based ink compositions, there is the problem of increased susceptibility to the occurrence of bronzing on glossy paper.
In addition, ink compositions tend to use easily handled aqueous vehicles, and lithium salts are used to maintain solubility in aqueous vehicles of various organic additives including colorants. More specifically, although lithium ions are used for the counter ions of the colorant, the use of a lithium salt is known to enhance water solubility of the colorant and increase resistance to the occurrence of bronzing on recorded articles (glossy paper) in cyan-based colorants. In addition, the use of lithium salts of compounds having a carboxyl group on a naphthalene backbone (such as 2-naphtoeic acid or 6-hydroxy-2-naphthoeic acid) is also known to be able to inhibit bronzing of cyan-based ink compositions (see, for example, JP-A-2004-263155). In any case, however, this results in an increased lithium ion concentration in the ink composition.
On the other hand, an ink composition for inkjet recording is filled into an ink cartridge having an ink housing chamber, and is used by installing in a printer. Various types of plastics are frequently used for the members that compose this ink cartridge because of their ease of processing and low cost. Although numerous additives are typically used in plastics, higher fatty acids in particular as exemplified by stearic acid are contained for use as lubricants. When these higher fatty acids bond with metal (such as K, Na or Li), a salt is frequently formed that is insoluble or poorly soluble in water or organic solvent. Consequently, there are cases in which higher fatty acids elute into the ink composition from plastic that has contacted the ink composition, these fatty acids then bond with metal in the ink composition thereby resulting in the precipitation of salts that are insoluble or poorly soluble in water and organic solvents over time, and eventually causing clogging and the like.
The inventors of the present invention confirmed that in the case the metal is lithium, salts that are insoluble or poorly soluble in water and organic solvents are formed to an even greater extent than in the case of other monovalent metal salts (such as sodium salts or potassium salts).
Namely, in the case of filling an ink composition containing a large amount of lithium ions as described above into the above-mentioned ink cartridge, when the lithium ions in the ink composition exceed a certain fixed concentration, lithium salts of fatty acids that are insoluble or poorly soluble in water and organic solvents precipitate over time, thereby causing problems such as clogging.
With respect to the above-mentioned problems, an inkjet recording apparatus is provided that defines the contents of fatty acids and/or fatty acid derivatives in the material of an ink tank that contacts an ink (see Japanese Patent No. 2874691). Alternatively, an inkjet recording apparatus is proposed that defines sodium ion concentration in an ink that contacts an ink tank containing a fatty acid and/or fatty acid derivative (see Japanese Patent No. 2696841). In addition, an aqueous ink that defines the fatty acid content in the ink and an inkjet recording method that uses that ink are proposed (see JP-A-9-3374).
However, in each of these examples of the prior art, the precipitation of fatty acid lithium salts that are insoluble or poorly soluble in water and organic solvents when using a colorant having various superior characteristics as described above is unable to be adequate prevented, thereby having the problem of being unable to realize both high printing quality such as that free of bronzing or storageablity of printed articles in the form of light resistance, gas resistance and the like, and prevention of printer clogging and the like during long-term storage while in contact with a plastic container or plastic member.